1. Field of the Invention
This invention relates to a dry etching method, and particularly to a dry etching method for processing a polycide layer, in which a poly Si layer and a high melting point silicide layer are formed in this order on an oxide film bed, by etching resist patterns selectively formed on said polycide layer into a mask. This invention can be used, for instance, for producing materials available in the field of electronics (to manufacture semiconductor devices or others). This invention can be embodied, for instance, as a dry etching technology using a side wall protection film in manufacturing high speed/high density semiconductor devices.
2. Prior Art
In recent years, various types of materials for electronics have become more minute and higher in density. As can be observed in the examples of VLSIs and ULSIs recently developed, in association with introduction of more minute design rules for semiconductor devices or others, it has becomes more and more important to etch a material layer under higher selectivity toward the bed.
For instance, when fabricating a gate electrode for MOS-FET, it is necessary to etch a silicon-based material layer such as a polycrystal silicon layer or a polycide film under the conditions where a high selectivity ratio to a thin gate insulating film made of silicon oxide (SiO.sub.2) can be achieved.
Also, when forming a contact in such a section as an impurities diffusing area formed in a semiconductor substrate, or a source drain area of a PMOS transistor, it is necessary to etch an insulating film between SiO.sub.2 layers under the conditions where a high selection ratio to such Si-based materials as a silicon substrate or a poly-silicon layer can be achieved.
However, such conditions selectivity to a substrate, anisotropy etching speed, low polluting characteristics, and low damaging characteristics are related to each other in a trade-off relationship, so that etching is carried out in actual processes appropriately adjusting the conditions in a range allowable for the actual operations.
Conventionally for etching layers made of Si-based materials such as monocrystal silicon, polycrystal silicon, refractory metallic silicide having a high melting point or polycide, a Freon (chlorofluorocarbon) -based gas represented by, for instance, Freon 113 (C.sub.2 Cl.sub.3 F.sub.3) is widely used as an etching gas. It is because the Freon-based gas has F and Cl as constituting elements in one molecule, so that etching due to both the radical reaction and the ion assist reaction can be carried out, and also a high anisotropy can be achieved while protecting the side walls with a carbon-based polymer deposited from the gas phase.
It has been pointed out, however, that the Freon-based gas, which is a representative etching gas for layers made of Si-based materials, is widely considered as adding to the destruction of the ozone layer around the earth, and it is expected that production and use of the materials will be prohibited in the near future. For this reason, now many efforts are made for development of other etching gasses which are available as alternatives to these sedimentary carbon-based gasses as well as for development of technologies enabling use of the alternative etching gasses to be developed in the future. For instance, the etching technology with a bromine gas (Br.sub.2) or a hydrogen bromide-based gas (HBr) has been developed (Refer to, for instance, the Japanese Patent Laid Open Publication No. 89310/1990).
In etching layers made of Si-based materials with a hydrogen bromide-based gas, side wall protection is carried out by a product from a reaction between the photoresist and Br, or between Si and Br. Also in over-etching where the priority is put on the selectivity to the substrate, the anisotropy, which has a trade-off relation with the selectivity to substrate becomes lower. For this reason, sometimes etching may be carried out under process conditions, which compensates for a decrease of anisotropy by using a more effective side wall protection film such as, for instance, a SiO.sub.2 film.
On the other hand, when an etching process ends, usually a resist layer used as an etching mask must be removed by means of ashing with an oxygen (O.sub.2) gas plasma. Then the side wall protection film used in etching a layer made of Si-based materials is also deposited on a side wall of the resist layer as a mask, only this side wall protection film, which can not be removed by the oxygen gas plasma, may remain in a belt-shaped area even after the resist layer has been removed by means of ashing.
As the side wall protection film left in the belt-shaped area adversely affects such factors as flatness of inter-layer films stacked in the subsequent processes, it must be removed before the inter-layer films are stacked. In the conventional technology, after anisotropic processing making use of a side wall protection film is carried out, the side wall protection film is removed by means of wet etching with a fluoric acid solution, but in case of wet etching with a fluoric acid solution the gate oxide film bed is simultaneously etched. This does not cause any specific problem if the minimum processing line width is fully large and also the gate oxide film bed is fully thick, but recently as the gate oxide film bed has been becoming thinner and thinner with increasing speed in association with technological advance in this field, this gives some influences over the gate oxide film bed to be etched, and sometimes the gate oxidized film bed is damaged.